Transparent armor panels may be comprised of multiple layers formed of different materials. For example, an armor panel may include an outer layer or strike face formed of glass, ceramic, or glass-ceramic material configured to receive an impact from a projectile. The ballistic performance of the armor panel may be improved by adding one or more layers to the backside of the strike face. For example, one or more layers of polycarbonate, polyurethane, polymethylmethacrylate (PMMA), or other material may be adhesively bonded to the backside of the glass layer (e.g., strike face) to improve the energy-absorbing capability of the armor panel.
Unfortunately, the bonding of the glass layer to the polycarbonate (or other) layer may induce the formation of residual stress in the panel assembly. The residual stress in the panel assembly may be caused by a difference in the coefficient of thermal expansion of the glass layer relative to the coefficient of thermal expansion of the polycarbonate layer. The residual stress may be induced in the panel assembly as a result of bonding the glass layer to the polycarbonate layer at a relatively high temperature (e.g., 180°-250° Fahrenheit) and then allowing the bonded panel assembly to cool to room temperature.
The residual stress may include tensile stress acting at the bondline of the panel assembly along one or more side edges of the panel assembly. The tensile stress along the side edges may result in a peel force located at the side edge and which may urge the glass layer and polycarbonate layer to peel away from one another at the bondline. Over time, such peel forces may result in delaminations in the bondline between the glass layer and the polycarbonate layer. Such delaminations may reduce the mechanical, ballistic, and optical performance of the panel assembly in the field.
Prior art attempts to reduce residual stress in composite panel assemblies include increasing the overall bondline thickness. Although generally effective in reducing residual stress between the layers of the panel assembly, the increase in bondline thickness unfortunately reduces the ballistic performance of the panel assembly. Furthermore, the increase in bondline thickness adds weight to the panel assembly which reduces the overall performance of the system (e.g., a vehicle or a structure) to which the panel assembly is mounted.
As can be seen, there exists a need in the art for a system and method for reducing residual stress in panel assemblies formed of materials having different coefficients of thermal expansion. Ideally, such system and method for reducing residual stress in panel assemblies is of simple construction, low cost, and lightweight.